This is the latest revision using a square cross section oring.  I made a version without a pressure release if you have a regulated CO2 source.Support is built into the model.  Use a screw driver in the gap to pop it out.I am working on two versions of "picnic caps" for dispensing straight out of the growler.  One will have a ball lock gas port and the other one will have a schrader valve gas port.Want one but don't have a printer?  I can print you one, download the cap you want then upload it here https://www.3dhubs.com/dallas/hubs/dfw-maker-depot/3dprint    Pick the low resolution for the order, I will print it at medium resolution and include a Buna-N oring.